Glass melting furnaces include a refractory surface or superstructure with an interior surface of the superstructure exposed to the harsh heated chemical environment of the furnace. A portion of the superstructure or roof, referred to in the industry as the crown of the furnace, is generally constructed from silica refractory bricks or silica bricks, which are referred to herein also as “bricks.” Oxy-fuel burners, used to melt glass-forming materials in alkali-based glass melting furnaces, generate high concentrations of alkali hydroxide vapor in the furnace atmosphere, such as sodium hydroxide (NaOH).
Alkali hydroxides form when water vapor, resulting from the combustion of fuel and oxidant, react with alkali oxides in the glass melt. These alkali hydroxides vaporize out from the molten glass, and can react with the furnace refractory. The alkali hydroxides such as NaOH can corrode the refractory superstructure, particularly where the residence time of the NaOH is unusually long, such that the effects of the NaOH and the partial pressure associated therewith in the closed furnace can compromise the structural integrity of the furnace by corrosion and subsequent melting of the crown of the furnace.
For example, known glass melting furnaces operate at temperatures at which a “glassy phase” can occur on the silica bricks exposed to the atmosphere above the glass being melted within the furnace. The glassy phase of the silica bricks will increase and the viscosity of this glassy phase decrease as the presence and concentration of the alkali, and to some extent water, increases within the atmosphere of the furnace and in contact with the refractory surface.
In addition, the presence of other oxides in the silica, for example, calcium oxide (CaO) and ferric oxide (Fe2O3) increases the quantity of the liquid phase formed on the silica bricks. Thereafter, a silica-saturated liquid phase will occur and “run” along the surface of the existing bricks to form droplets of silica liquid which drip and ultimately flow down the surface of the silica crown. As the residence time of the NaOH increases, and concurrently the concentration of the alkali increases, the liquid phase will continue to dissolve more of the silica bricks in the furnace refractory superstructure, and will ultimately erode and compromise the furnace crown structure.
Such disadvantages of the alkali presence are known and disclosed in a publication entitled “Degradation of Crown Silica Refractories in Glass Tank Melters,” M. Velez et al., University of Missouri-Rolla, Rolla, Mo., published for the 61st Conference on Glass Problems, Ohio State University, Columbus, Ohio, Oct. 17-18, 2000, (Conference Proceedings edited by Charles H. Drummond III), pages 79-89. The corrosion resistance of the silica bricks is reduced under conditions where oxy-fuel melt applications are employed in the furnace.
It is therefore desirable to have a glass melting furnace constructed to purge or exhaust the NaOH, or at the least substantially reduce the residence time of the NaOH at the refractory superstructure so that during the high heat melting of the glass product, the silica bricks which comprise the crown of the furnace are not corroded.